The present invention relates to cutting (for instance of metals) by jets of liquid including entrained abrasive particles.
The use of high velocity water jets containing entrained abrasive particles for cutting purposes has been known since about 1980. Known cutting water jet systems fall into one of two categories: Abrasive water jet (AWJ) systems and Abrasive suspension jet (ASJ) systems.
AWJ systems typically supply water at extremely high pressure (in the order of 150 to 600 MPa) to a nozzle. A typical AWJ nozzle 10 is shown in FIG. 1. The nozzle 10 includes a small orifice 12 (0.2 to 0.4 mm diameter) which leads into a mixing chamber 14. Water thus flows through the mixing chamber 14 at a high velocity.
Small grains of abrasive material, typically garnet, are supplied to the chamber, generally by a gravity feed through a hopper 16. The high water velocity 18 creates a venturi effect, and the abrasive material is drawn into the water jet.
The water jet then flows through a length of tubing known as a focussing tube 20. The passage of water and abrasive through the focussing tube 20 acts to accelerate the abrasive particles in the direction of water flow. The focussed water jet 22 then exits through an outlet 24 of the focussing tube. The water jet 22—or, more accurately, the accelerated abrasive particles—can then be used to cut materials such as metal.
The energy losses in the nozzle 10 between the orifice 12 and the outlet 24 of the focussing tube 20 can be high. Kinetic energy of the water is lost by the need to accelerate the abrasive material, and also to accelerate air entrained by the venturi. Significant frictional losses occur in the focussing tube 20, as abrasive particles ‘bounce’ against the walls of the tube. This results in energy loss due to heat generation. As an aside, this phenomenon also results in degradation of the focussing tube 20, which typically needs replacing after about 40 hours' operation.
Known AWJ systems are therefore highly inefficient.
ASJ systems combine two fluid streams, a liquid (generally water) stream and a slurry stream. The slurry contains a suspension of abrasive particles. Both liquid streams are placed under a pressure of about 50 to 100 MPa, and are combined to form a single stream. The combined stream is forced through an orifice, typically in the order of 1.0 to 2.0 mm diameter, to produce a water jet with entrained abrasive particles.
ASJ systems do not suffer from the same inefficiencies as AWJ systems, as there is no energy loss entailed in combining the two pressurised streams. Nonetheless, known ASJ systems are of limited commercial value. This is partly because ASJ systems operate at significantly lower pressures and jet velocities than AWJ systems, limiting their ability to cut some materials.
ASJ systems also evidence significant difficulties in operation, primarily due to the presence of a pressurised abrasive slurry, and to the lack of effective means to provide control over its flow characteristics. The parts of the system involved in pumping, transporting and controlling the flow of the abrasive slurry are subject to extremely high wear rates. These wear rates increase as the pressure rises, limiting the pressure at which ASJ systems can safely operate.
Of possible greater significance are the practical difficulties inherent in starting and stopping a pressurised abrasive flow. When used for machining, for instance, a cutting water jet must be able to frequently start and stop on demand. For an ASJ system, this would require the closing of a valve against the pressurised abrasive flow. Wear rates for a valve used in such a manner are extremely high. It will be appreciated that during closing of a valve the cross-sectional area of flow decreases to zero. This decreasing of flow area causes a corresponding increase in flow velocity during closing of the valve, and therefore increases the local wear at the valve.
In a typical industrial CNC environment, cutting apparatus can be required to start and stop extremely frequently. This translates to frequent opening and closing of valves against pressurised abrasive flow, and rapid wear and deterioration of these valves. As a result, the use of ASJ systems for CNC machining is known to be inherently impractical.
ASJ systems have found use in on-site environments, such as oil-and-gas installations and sub-sea cutting, where the cutting required is largely continuous. ASJ systems have not been commercially used in industrial CNC machining.
FIGS. 2a and 2b show schematic representations of known ASJ systems. In a basic single stream system 30, as shown in FIG. 2a, a high pressure water pump 32 propels a floating piston 34. The piston 34 pressurises an abrasive slurry 36 and pumps it into a cutting nozzle 38.
A simple dual-stream system 40 is shown in FIG. 2b. Water from the pump 32 is divided into two streams, one of which is used to pressurise and pump a slurry 36 by means of a floating piston 34 in a similar manner to the single stream system 30. The other stream, a dedicated water stream 35, is combined with a pressurised slurry stream 37 at a junction prior to the cutting nozzle 38.
Both of these systems suffer from the problems outlined above, and result in very high valve wear rates. Other problems include an inconsistent cutting rate due to extreme wear in the tubes and nozzle.
In continuous flow systems, such as sub-sea applications, known ASJ systems are limited in that operating properties of the system such as the system pressure, and the ratio of water to abrasive slurry, must be set prior to operation, or even at the time of manufacture.
An alternative arrangement is proposed in U.S. Pat. No. 4,707,952 to Krasnoff. A schematic arrangement of the Krasnoff system 50 is shown in FIG. 3a. The Krasnoff system is similar to the dual-stream system 40, with the difference being that mixing of the water stream 35 and slurry stream 37 takes place in a mixing chamber 52 within the cutting nozzle 38.
A more detailed view of the mixing chamber 52 of Krasnoff is shown in FIG. 3b. The nozzle 38 provides a two-stage acceleration. Firstly, the water stream 35 and the slurry stream 37 are accelerated through independent nozzles leading into the mixing chamber 52. Then the combined water and abrasive stream is accelerated through the final outlet 54.
The Krasnoff system is arranged to operate at a pressure of about 16 MPa, significantly lower than other ASJ systems. As such, the impact of the slurry stream 37, whilst still damaging to valves, results in reduced valve wear rates than in higher pressure systems. The corollary is, of course, that the power output of the Krasnoff system is even lower than other ASJ systems, and thus its commercial applications are small. The applicant is not aware that the Krasnoff system has ever been commercially applied.
The present invention seeks to provide a system for creating a high pressure water jet with entrained abrasive particles which overcomes, at least in part, some of the above mentioned disadvantages of above AWJ and ASJ systems.